MyD88-dependent toll-like receptor signalling is not a requirement for fetal islet xenograft rejection in mice

Background: Rejection of pancreatic islet xenografts in mice shares immunopathological features with a Th1-associated delayed-type hypersensitivity (DTH) reaction. The aim of the present study was to investigate the mechanism of acute cellular xenograft rejection in a strain of mice with a targeted gene disruption of the toll-like receptor (TLR) signal adaptor protein MyD88. These mice have been shown to have markedly impaired Th1 immunity. Methods: The MyD88-/- and normal mice were transplanted with 2 mul of fetal porcine islet-like cell clusters (ICC) under the left kidney capsule. On days 3, 6 or 12 after transplantation the mice were killed and the grafts either prepared for immunohistochemistry or real-time quantitative reverse transcriptase polymerase chain reaction (RT-PCR). The number of remaining ICC and infiltrating cells with different phenotypic characteristics was assessed semi-quantitatively. Grafts used for quantitative RT-PCR were analysed for content of murine mRNA of interferon (IFN)-gamma, interleukin (IL)-12p40, IL-4 and IL-10. Results: On day 3, the rejection process was initiated in both MyD88-/- and normal mice as characterized by a moderate infiltration of F4/80(+) and MAC-1(+) macrophages and occasional CD3(+) and CD4(+) cells. Expression of IFN-gamma and IL-12p40 was lower but still detectable in the MyD88-/- mice, when compared with control animals. By day 6, rejection was almost completed in all animals with only few ICC remaining. 12 days after transplantation all grafts were completely destroyed and heavily infiltrated by macrophages. Moderate numbers of CD3(+) and CD4(+) and occasional CD8+ cells were also present. Conclusions: Islet xenograft rejection was found to persist in MyD88-/- mice. Despite a relatively lower expression of the Th1-associated cytokines IFN-gamma and IL12-p40 within the xenograft area, both the time course and morphological pattern of the rejection were essentially similar to that found in normal animals. Hence, MyD88-dependent TLR signalling does not appear to be a crucial component of acute cellular xenograft rejection.